I have a large document, that I have decided I want to make the text smaller.

However there are loads of different text sizes and it would take an age selecting them all and making them bigger.

So I was just wondering if there was an option to maybe "-1" to all font sizes (e.g. so all font sizes 18 in the document would become 17, and all font sizes 12 would become 11), or whether something like that was planned, or if there was another way to do it. .

OpenOffice.org Writer: Use styles to update paragraphs throughout a long document

If you use styles like heading 1, heading 2, text body etc. when writing your document, you can change the formatting of the various styles all at once wherever they appear in the document. You can define your own paragraph or character styles as well as using the built in ones. In addition, you can import styles from another document so that your formatting can be changed easily and appear consistent across documents. If you have used styles with MS Word or formatted HTML documents using a style-oriented composer (Mozilla, FrontPage etc.), it should be pretty easy to figure out.

If your document is formatted with styles, then format a paragraph labeled with the appropriate style like you want it. Make sure the whole paragraph is highlighted. Activate the styles palette with Format > Styles and Formatting F11.
Hold down the button at the extreme top right with the tooltip New Style from Selection. From the drop down menu select Update Style. Every paragraph with that style will be updated with the same font, font size, borders etc.

Some styles inherit characteristics from other styles. For example, Text Body settings are partially determined by the settings for Default style unless you set the Text Body style otherwise.

For more information about styles, see Help > OpenOffice.org Help F1, click Find, and search for Styles in Writer.

The only way I can think of to do this is to edit the XML text of the document. You could probably make a little script to reduce all the point sizes by 20%. In fact, I just tried this and it seems to have worked: